Device for detection and control of leakage and excessive current flow

ABSTRACT

A protective circuit device for connection between an associated electrical power supply and an associated electrical load which detects excessive current drawn by the load. Solid state devices in the protective device are rendered conductive or nonconductive in response to a signal produced by a current transformer. The amplified signal is applied to the gate of a silicon controlled rectifier which shorts power going to a unijunction transistor oscillator. The shorting action terminates a pulse trigger supply to the solid state devices to render them nonconductive. In one embodiment, the protection device also detects electrical leakage from the load by means of a differential transformer which produces an error signal when there is current imbalance between conductors from the line supply. The amplified differential transformer secondary voltage is applied to the gate of the silicon controlled rectifier to cause the solid state devices to become nonconductive.

United States Patent Ambler et al.

[54] DEVICE F OR DETECTION AND CONTROL OF LEAKAGE AND EXCESSIVE CURRENTFLOW [72] lnventors: Edward Curtis Ambler, Newington; Walter R. Bush,West Simsbury; Andrew E. Scoville, Ellington, all of Conn.

[73] Assignee: The Stanley Works, New Britain, Conn. [22] Filed: Dec.28, 1970 [2]] App]. No.: 101,748

[52] U.S.Cl ..3l7/l8 D,3l7/27 R,3l7/33 SC [5 1] Int. Cl. ..H02h 3/28[58] Field of Search ..3l7/33 SC, 18 D, 27

[56] References Cited UNITED STATES PATENTS 3,408,558 10/1968 Petersonet al ..3l7/33 SC 3,324,352 6/1967 Hover ..3l7/33 SC 3,566,189 2/1971Wilson et a]. ..3l7/18 D 3,525,903 8/1970 Morris et al ..3l7/l8 D [451June 6, 1972 Primary Examiner-James D. Trammell Att0rney-Peter L. Costas5 7] ABSTRACT A protective circuit device for connection between anassociated electrical power supply and an associated electrical loadwhich detects excessive current drawn by the load. Solid state devicesin the protective device are rendered conductive or nonconductive inresponse to a signal produced by a current transformer. The amplifiedsignal is applied to the gate of a silicon controlled rectifier whichshorts power going to a unijunction transistor oscillator. The shortingaction terminates a pulse trigger supply to the solid state devices torender them nonconductive.

In one embodiment, the protection device also detects electrical leakagefrom the load by means of a differential transformer which produces anerror signal when there is current imbalance between conductors from theline supply. The amplified differential transformer secondary voltage isapplied to the gate of the silicon controlled rectifier to cause thesolid state devices to become nonconductive.

14 Claims, 2 Drawing Figures I22 I24 I g I llfl I 1/4 I ///0 I I I I fim I @9 LOAD 1312 l I I me I ,//z I //6 I 120 I? 1 4 1 DEVICE FORDETECTION AND CONTROL OF LEAKAGE AND EXCESSIVE CURRENT FLOW BACKGROUNDOF THE INVENTION Increasing use of electrical power for many diverseapplications along with increasing concern about hazards associated withsuch use has produced requirements for detection and control of excesscurrent flow and leakage from electrically powered devices. Conventionalcircuit breakers provide an overload portion adapted to provide circuitinterruption when there is a continuous current flow at a moderate levelabove the nominal rating of the circuit breaker and, in addition, ashort circuit" trip portion designated instantaneously to interrupt acircuit if it detects a current flow considerably above the nominalrating. Generally such devices are mechanical in nature with movingparts which are relatively slow moving and have a reliability factorless than that provided by presently available static switching devices.Moreover, such protective devices that do have overload and shortcircuit" protection generally do not include protection againstline-to-ground leakage paths when the current flowing is less than thenominal rating of the device.

Considerable difficulty has also been experienced with devices providingonly one of these forms of protection because there is a continuingdanger of electrocution as well as fire since any one of these types ofcircuit malfunction constitutes a hazard and may exist without one ofthe other types.

The utilization of circuit protective devices having both forms ofprotection adjacent the load is desirable to provide circuit protectionwhile minimizing interruption of power to other devices, and suchplacement is particularly important where the device includes groundfault protection in order to provide a system which avoids nuisancetripping problems. When ground fault devices are used at a panel boardor switchboard to limit ground fault current, the cumulative leakage ofthe various electrical devices connected to that branch circuit oftenresults in a ground fault trip although no one of the devices hasleakage sufficient to be hazardous. If the set point of the ground faultprotection device is raised to a sufficient level to allow current flowand avoid nuisance tripping, the leakage from a particular device may behazardous to personnel or likely to cause fire or explosion although thehigher branch circuit ground fault limit has not yet been reached.Utilization of devices having the several features adjacent individualloads as opposed to placement in branch circuits carrying a plurality ofdevices has been deterred in large part because of the relatively highinitial and operating costs for devices heretofore known and/or becauseof the size involved for the mechanical elements heretofore employedmost generally in such devices.

It is an object of the present invention to provide a novel electricalprotective circuit device which is capable of responding reliably tocurrent flow in excess of a desired level and which may be manufacturedand operated at low cost to facilitate use in an electrical circuitadjacent the electrical load.

It is also an object to provide such a protective circuit device whichalso is capable of sensing very small ground faults producing a leakageof current to ground below the overload current rating of the device.

Another object is to provide such a protective circuit device which maybe simply and economically constructed and adjusted to open a circuit inresponse to dangerous ground fault conditions while avoiding nuisancetripping.

A further object is to provide such a device having a very rapid circuitopening action to insure the protection will be provided quickly enoughto prevent electrocution and to minimize the likelihood of fire orexplosion, and which is failsafe in design to minimize the impact ofcomponent failure.

SUMMARY OF INVENTION It has now been found that the foregoing andrelated objects can be readily attained in a protective circuit devicefor detection and control of leakage from an associated electrical loadand excessive current flow from a line power supply to an associatedelectrical load including a pair of terminals for connection to anexternal associated alternating current line power supply and a secondpair of terminals for connection to an external associated load. Thereis at least one solid state device connecting one of the first pair ofterminals and one of the second pair of terminals and the solid statedevice has conductive and nonconductive states. Means for changing theprevailing state of the solid state device is provided which includesmeans generating a pulse trigger signal, which is transmittable to thesolid state device to maintain one of such states, and the state of thesolid state device changes upon termination of the pulse trigger signal.A current transformer has a core of magnetically susceptible metal, onlya single primary winding connected in series between one of the firstpair of terminals and one of the second pair of terminals, and asecondary winding which provides an output during operation which is afunction of current flow in the primary winding and below a valueestablished by the nominal rating of the protective circuit device.Means is connected to the secondary winding responsive to an output inexcess of the value resulting from current drawn by the load in excessof the nominal rating of the protective circuit device and is operativeto terminate the pulse trigger signal. As a result, excess current willproduce an output in excess of the established value and the responsivemeans will terminate the pulse trigger signal to change the state of thesolid state device to open the circuit controlled thereby.

Preferably, the means connected to the secondary winding operatessubstantially instantaneously upon occurrence of an output substantiallyin excess of the nominal rating and, upon occurrence of an output inexcess of the nominal rating of lesser magnitude, only after arelatively extended period of time. Most desirably the means connectedto the secondary winding includes a resistance and a capacitanceconnected in parallel relationship and a second adjustable resistance topermit adjustment of the value of the output which will operate themeans to terminate the pulse trigger signal. The means for changing theprevailing state of said solid state device includes a selectivelyconductive silicon controlled rectifier to prevent generation of thepulse trigger signal.

Desirably, the means connected to the secondary winding includes arectifier and one stage of solid state amplification. A second solidstate device preferably is connected between the other of the first pairof terminals and other of the second pair of terminals, with the secondsolid state device having conductive and nonconductive states dependentupon the means for changing the prevailing state of the first-mentionedsolid state device.

The solid state devices may be triacs and the means generating a pulsetrigger signal may comprise a unijunction transistor oscillator. Themeans generating a pulse trigger signal may include a bridge rectifierfor rectifying the associated alternating current line power supply. Inits most desirable aspect, the protective circuit device will includeground fault detection capabilities. As such, it will incorporate adifferential transformer having a core of magnetically susceptiblemetal, a pair of like primary windings disposed about the core, each ofthe primary windings being connected in series between one of the firstpair of temiinals and one of the second pair of terminals. The pair ofprimary windings under balanced current conditions therein produces abalanced total magnetomotive force so that the net magnetic flux in thecore is zero. Under leakage current conditions, the pair of primarywindings produce an unbalanced magnetomotive force to create a voltagein the secondary winding of the differential transformer. Meansresponsive to the voltage in the secondary winding of the differentialtransformer terminates the pulse trigger signal,

whereby the state of the solid state device may be changed to open thecircuit controlled thereby.

Most preferably the means responsive to the voltage in the secondarywinding for terminating the pulse trigger signal includes a rectifierand a stage of solid state amplification for this voltage.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of aprotective circuit device embodying the present invention; and

FIG. 2 is a circuit diagram of another embodiment of the inventionadditionally including ground fault protection.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Turning now indetail to FIG. 1, of the attached drawing, therein illustrated is asource of alternating current line power and a load 16 connected inseries relation to a protective circuit device embodying the presentinvention. The device has input terminals 11 for the protective circuitpower supply 12, and a sensing and isolation circuit generallydesignated by the numeral 14. Conductors 17, 18 provide a current flowpath for line power between input terminals 11 and a pair of outputterminals 19 across which the load 16 is connected.

The power supply 12 includes resistors 20, 21 connecting the line powerto a bridge rectifier including diodes 22, 24, 26, 28. The resistor 36is connected to the common junction of diodes 22 and 28, and theresistor 34 is connected to the common junction points of diodes 24 and26 which are connected to zener diodes 38, 40 to provide voltageregulation of the bridge output. The zener diodes 38, 40 are connectedin series to provide" voltage regulation which may be preciselyestablished and which is not significantly affected by changes in inputvoltage or junction temperature. The rectified and regulated voltageoutput of the power supply 12 is carried by conductors 42, 44 to thesensing and isolation circuit 14.

Interposed in the line conductor 17 is a current transformer primarywinding 45, which is wound about a core 47 and cooperates with thecurrent transformer secondary winding 49; as will be appreciated, theoutput of a secondary winding is a function of the current flow in theprimary winding and thus in the line conductor 17. A resistor 51 isconnected in parallel with the primary winding 45 for apportioningcurrent in the conductor 17 so as to reduce the requirements for thecurrent transformer primary winding 45, to lower the line losses, toconveniently adjust sensivity and to conveniently lower the currentlevels for design of the associated trigger circuits. On one side of thesecondary winding 49 is connected the conductor 44 and on the oppositeside thereof are connected the resistor 53 and the diode 55, which areconnected in series with the base 57 of the transistor indicatedgenerally by the numeral 59.

Connected in parallel between the potential of base 57 and the conductor44 is a network consisting of the capacitor 61 and the resistor 63 whichprovide a desired time versus current relationship; more specifically,the charging and discharging of the capacitor 61 as affected by currentflow through the resistor 63 to the conductor 44 varies the signalapplied to base 57. When the critical level signal on base 57 isreached, the transistor 59 will transmit a signal from the emitter 61thereof to the resistor 74 and thence to the gate 78 of a siliconcontrolled rectifier (SCR) 80.

The collector 63 of the transistor 59 is connected to the conductor 42by means of the resistor 65 which limits the current flowing thereto.The resistor 74 controls the current supplied to the gate 78 of the SCR80 and the capacitor 76 connected .between the gate 78 and the conductor44 eliminates spurious noise spikes. The SCR 80 has a cathode 82 whichis connected to the conductor 44 and an anode 84 which is connected inseries with the resistor 86 to the conductor 42.

Connected to the anode 84 of SCR 80 are the filter or capacitor 88 andthe emitter 90 of the unijunction transistor generally designated by thenumeral 92. The unijunction transistor 92 has a first base 94 connectedto the pulse transformer primary winding 102, which has connected acrossit a diode for wave shaping. The second base 96 of the transistor 92 isconnected to the conductor 42 through the resistor 98 which controlsbiasing of the unijunction transistor 92.

The transformer primary winding 102 is disposed about the core 104 andcooperates with the secondary windings 106 and 108 which are in turnconnected through the resistors 110, 112 to the gates 118, of the triacsgenerally designated by the numerals 114, 116. The terminals 122, 124 ofthe triac 114 are interposed in conductor 17, and a signal operating onthe gate 118 will render the material of the triac between the tenninals122, 124 conducting or nonconducting depending upon the arrangementemployed. Similarly, the terminals 124, 128 of the triac 116 areinterposed in conductor 18 and the material therebetween is renderedconducting or nonconducting by the signal applied to the gate 120. Anindicator light 130 is connected across conductors 17, 18 to sense thecondition of the triacs 114, 116, i.e., whether they are conducting. Thecapacitor 132 is positioned in parallel to the indicator light 130 andthe load 16 is connected between the terminals 19, 19 of the conductors17, 18.

Testing is effected by means of the momentary push button switch 129connected in series with the resistor 131 between conductors 17, 18 toestablish a mementary current flow path whereby current flows throughcurrent transformer primary winding 45. The resistor 131 limits themaximum current draw during the test operation.

Turning now in detail to FIG. 2 of the attached drawing, the componentsof FIG. 1 are included therein although somewhat different inarrangement; the same reference numerals will be employed whereappropriate. In this embodiment, other components have been added toprovide ground fault protection capabilities in the protective circuitdevice.

Interposed respectively in the line current conductors 17, 18 are twoidentical transformer primary windings 46, 48 disposed about a commoncore 52 of magnetically susceptible metal together with a secondarywinding 50, thus forming a differential current sensing transformer. Thecore 52 is preferably of ferrite construction or tape wound and theprimary windings 46, 48 are normally wound about the same portion ofcore 52in the same direction with the same number of turns.

Equal current in the line conductors 17, 18 and therefore equal currentin the primary windings 46, 48 results in magnetic flux being created ineach primary winding which is of the same magnitude and in oppositedirections because of the opposite directional current flow in theconductors 17, 18. Accordingly, there is zero magnetic flux because theflux of the two primary windings 46, 48 cancel out, and no electromotiveforce is induced in the secondary winding 50.

In the event that there is leakage from the load 16, the current flow inthe primary windings 46, 48 will not be equal and a net magnetic fluxwill be produced which will cause a voltage to appear across thesecondary winding 50. That voltage is the signal which is then amplifiedand utilized to trigger the circuitry which isolates the electrical load16 in response to a ground fault.

The capacitor 54 is connected across the secondary winding 50 and isalso connected on one side to the reference level of conductor 44 and onthe other side to the diode 56 which rectifies the output of thesecondary winding 50. Connected to the other side of the diode 56 inseries relation to each other are the resistor 58 and the capacitor 60;the capacitor 60 is an integrator and filter for the output of thedifferential transformer secondary winding 50 and the resistor 58 limitscurrent flow through the diode 56 into the capacitor 60 during circuitstart-up conditions. The resistor 62 is positioned in parallel betweenthe resistor 58 and the conductor 44 for the purpose of providingsensitivity adjustment for the circuit.

Additionally connected to one side of the diode 56 is the base 66 of thetransistor 64 which is of the n-p-n type. The collector 70 of thetransistor 64 is connected to the conductor 42 and the emitter 68thereof transmits the amplified current leakage signal to the resistor74. The connections to the resistor 74 have been described previouslywith respect to the overcurrent portions of the circuit as regards theembodiment of FIG. 1; thus, the signal is imparted to the siliconcontrolled rectifier 80.

To provide for test operation of the ground fault circuit the momentarypush button switch 134 is connected in series with the resistor 136between the conductors 17, 18. The momentary closing of the push button134 allows passage of current through the primary winding 48 but it doesnot allow passage through the other primary winding 46, therebysimulating a difference in current between line conductors 17 and 18.The resistance value of the resistor 136 is sufficiently large so thatthere is insufiicient current load to cause operation of the overcurrentportions of the circuit. During the test of the overcurrent portions ofthe circuit with the momentary push button 129 and the resistance 131,equal currents flow in the differential transformer primary windings 46,48. The level of current flow is sufficient to operate the overcurrentportions of the circuit but the ground fault circuitry is not actuatedbecause the current flow in primary windings 46, 48 is equal. The light130 is connected to the terminals 124, 128 of triacs 114, 116, and whenlighted, indicates that both triacs are in the conductive state. Thecapacitor 132 is positioned in parallel to the indicator light 130 tomaintain normal triac operation in the absence of other loads. Toprovide for resetting of this embodiment a reset switch 140 isinterposed in the conductor 17 between the input terminal 1 1 and theresistor 20.

OPERATION OF THE DEVICE Turning first to FIG. 1 and to the operation ofthe embodiment which provides overcurrent protection only, connection ofa source of line power to the input terminals 11, 11 energizes the lineconductors 17, 18 and power supply 12.

During one-half of each line power cycle, the current will flow throughresistor 20, diode 22, resistor 36, zener diodes 40 and 38, resistor 34,diode 26 and thence to resistor 21 back to the line power source 10.During the other half of each line power cycle, current will flowthrough resistor 21, diode 28, resistor 36, zener diodes 40 and 38,resistor 34, diode 24, and then to resistor back to the dource of linepower 10. The capacitors 30, 32 are connected in series between thejunction points of the diodes 22, 28 and the junction point of thediodes 24, 26 to provide a filtering of the bridge output.

The output of current transformer secondary winding 49 is rectified bythe diode 55 whereupon the signal is altered by the RC networkconsisting of the resistor 63 and the diode 61. The current path throughresistor 63 significantly reduces the charging rate for the capacitor 61during periods when the signal from diode 55 is small but, when thatsignal becomes large in magnitude, the current flow through the resistor63 is relatively small and capacitor 61 will charge more readily,thereby allowing a signal to pass to the base 57 of the transistor 59.

The signal applied to the base 57 determines the output from the emitter61 of the transistor 59. That output is adjusted by means of the fixedresistor 74 and filtered by the capacitor 76 before passing to the gate78 of the SCR 80. The signal applied to the gate 78 determines whetherSCR 80 is conductive or nonconductive between the anode 84 and thecathode 82. When SCR 80 is conductive, the capacitor 88 is never chargedthrough the resistor 86 and therefore the unijunction transistor 92 doesnot function and the triacs 114, 116 will be nonconductive. When nosignal is applied to the gate 78 of SCR 80, it will not be conductivebetween the anode 84 and the cathode 82, and the capacitor 88 may becharged through the resistor 86 until the firing point of theunijunction transistor 92 is reached in a repetitious cyclical patternto produce a pulse train in the pulse transformer primary winding 102and thereby the secondary windings 106, 108 which causes triacs 114, 116to become and stay conductive.

Current flowing in conductor 17 is divided between the resistor 51 andthetransformer primary winding 45, which is coupled to the secondarywinding 49. The output of the secondary winding 49 passes through theresistor 53 which limits its level before it passes through the diode 55which rectifies the signal before it reaches the RC network consistingof the capacitor 61 and the resistor 63. The combination thereofprovides the desired rapid response to very high current flow relativeto a predetermined level or nominal rating for the protective device anda slower response to current only somewhat above the desired level.

The signal is then passed to the base 57 of the transistor 59 to controlthe signal from the emitter 61 thereof, which, in turn, is limited bythe resistor 74 and filtered by the capacitor 76 before passing to thegate 78 of SCR 80. The values of the various components are selected toinsure that normal current flow in the line conductor 17 does nottrigger conduction of the SCR 80. Upon reaching a predetermined criticallevel, however, the current flow will cause the gate signal applied tothe gate 78 of SCR 80 to reach a sufficient level to make that deviceconductive.

When the gate signal applied to the gate 78 is not of a level sufficientto make the SCR 80 conductive, but there is an overcurrent of lessermagnitude, the potential between the conductors 42 and 44 will cause thecapacitor 88 to charge through the resistor 86 to the firing point levelof the unijunction transistor 92. The resistor 86 allows sufi'rcientcurrent flow to charge capacitor 88 when SCR 80 is nonconductive andlimits the current drain of the power supply when it is conductive. Whenthe unijunction transistor 92 fires, the capacitor 88 is dischargedthrough the emitter to the base 94 and the pulse transformer primarywinding 102. The cyclical repetition of this series of events creates apulse train which appears not only at the primary winding 102, but alsoat the secondary windings 106, 108, and thereby at the gates 118, of thetriacs 114, 116, causing them to remain in a conductive state. The useof an oscillator in this manner as opposed to a fixed bias results in aminimum power consumption to control the triacs 1 14, 1 16.

When the signal applied to gate 78 of SCR 80 reaches the minimum levelrequired to make that device conductive between the anode 84 and cathode82, the capacitor 88 will no longer charge. Accordingly, the unijunctiontransistor 92 will not reach its firing level and no pulse train isproduced in pulse transformer windings 102, 106, 108. The triacs 1 14,116 will not be conductive as a result and output terminals 19 will beisolated from the input terminals 11.

The operation of the circuit protective device shown in FIG. 2 issimilar to that of FIG. 1 except that the ground fault detectioncomponent circuitry may apply a signal to the gate 78 of SCR 80. Morespecifically, a difference in current flow between the primary windings46, 48 produces a voltage across the secondary winding 50 of thedifferential transformer. This potential is rectified by the diode 56,integrated and filtered by the capacitor 60 and amplified by thetransistor 64 before passing to the gate 78 of the SCR 80, thusproducing operation of the protective circuit device to open thecircuit.

Normal operation of either test switch 134 or 129 will cause SCR 80 tobecome conductive and cause triacs 1 l4, 1 16 to go into a nonconductivestate. The characteristics of the SCR 80 are such that, upon becomingconductive in response to the gate signal, it will continue to conductso long as current flow continues between its anode 84 and cathode 82.In normal operation, the only way current flow may be interruptedbetween these points is by disconnecting the source of line power. Inthe embodiment of FIG. 1, this will entail disconnecting the conductors17, 18 from the line power source 10, as for example, by removing a plugfor the load providing device from a conventional house receptacle. Inthe embodiment of FIG. 2, the reset switch 140 in the conductor 17 maybe opened temporarily to disconnect the elements of the device from thesource 10 of line power to effect resetting.

When disconnected from the line power source 10, the SCR 80 of theillustrated embodiments will return to its nonconductive state.

Although the present invention has been illustrated and described withrespect to a single phase circuit protection device, it will be readilyappreciated that it is also adapted to polyphase circuits. Generallythis will entail a separate transformer in each phase to provide theovercurrent protective circuitry in order to ensure adequatesensitivity. In such polyphase circuitry, there should be coactionbetween components in each of several phases to produce simultaneousopening. Moreover, it is apparent that significantly differentarrangements of the components and different components may be usedwithout departing from the spirit of the invention.

By the arrangement of components illustrated, the protective device maybe calibrated and set to respond to predetermined levels of overcurrentand ground fault leakage, thus avoiding spurious action while at thesame time ensuring essentially instantaneous action. The nominal ratingof the protective device will determine the general values for thevarious components and adjustable components may provide precisecalibration of the assembly. The capacitor charging the mechanism willprovide operation when the overcurrent is relatively small butsustained, and a high overcurrent will produce instantaneous operation.When employed in conjunction with ground fault protective circuitry, aground fault leakage will produce operation although it does not producea current flow in excess of the nominal rating.

Thus, it can be seen from the foregoing detailed specification anddrawings that the present invention provides a highly effective circuitprotective device which is capable of sensitive response to circuitovercurrent and ground fault irregularities and which is relativelyinexpensive to manufacture and operate. By being particularly adaptedfor use near the point where the electrical power is used (at the load)it avoids nuisance tripping and at thesame time quickly and accuratelyresponds to overcurrent and small leakage problems.

It will also be seen that a positive signal is required to render thesolid state device conductive and, therefore, the failure of mostcomponents will cause the circuit to interrupt power accordingly, thedesign is of a fail-safe nature. The relative simplicity of thecircuitry makes the device reliable and simple to assembly, and thecomponents may be selected to provide relatively long-lived,trouble-free operation.

We claim:

1. A protective circuit device for detection and control of leakage froman associated electrical load and excessive current flow from a linepower supply to an associated electrical load, comprising:

a. a first pair of terminals for connection to an external associatedalternating current line power supply;

b. a second pair of terminals for connection to an external associatedload;

c. at least one solid state device connecting one of said first pair ofterminals and one of said second pair of terminals, said solid statedevice having conductive and nonconductive states; means for changingthe prevailing state of said solid state device including meansgenerating a pulse trigger signal, said pulse trigger signal beingtransmittable to said solid state device to maintain one of such states,the state of said solid state device changing upon termination of pulsetrigger signal;

e. a current transformer having a core of magnetically susceptiblemetal, and only a single primary winding connected in a series betweenone of said first 'pair of terminals and one of said second pair ofterminals, and a secondary winding, said secondary winding providing anoutput during operation which is a function of current flow in theprimary winding and below a value established by the nominal rating ofthe protective circuit device;

f. means connected to said secondary winding and responsive to an outputin excess of said value resulting from current drawn by the load inexcess of the nominal rating of the protective circuit device andoperative to terminate said pulse trigger signal whereby excess currentwill produce an output in excess of said value and said responsive meanswill terminate the pulse trigger signal to change the state of saidsolid state device to open the circuit controlled thereby, and saidresponsive means including a selectively conductive silicon controlledrectifier to prevent generation of said pulse trigger signal which isnormally nonconductive and becomes conductive upon a predeterminedoutput in excess of said value to terminate said pulse trigger signaland which latches in said conductive state.

2. The protective circuit device of claim 1 wherein said means connectedto said secondary winding operate substantially instantaneously uponoccurrence of an output substan' tially in excess of said nominal ratingand, upon occurrence of an output of an excess of lesser magnitude, onlyafter a relatively extended period of time.

3. The protective circuit device of claim 2 wherein said means connectedto said secondary winding includes a resistance and a capacitenceconnected in parallel relationship.

4. The protective circuit device of claim 3 wherein said means connectedto said secondary winding includes a second adjustable resistance topermit adjustment of the value of the output which will operate saidmeans to tenninate said pulse trigger signal.

5. The protective device of claim 1 wherein said means connected to saidsecondary winding includes a rectifier.

6. The protective circuit device of claim 5 wherein said means connectedto said secondary winding includes one stage of solid stateamplification.

7. The protective circuit device of claim 1 wherein a second solid statedevice is connected between the other of said first pair of terminalsand other of said second pair of terminals, said second solid statedevice having conductive and nonconductive states dependent upon saidmeans for changing the prevailing state of said first-mentioned solidstate device.

8. The protective circuit device of claim 7 wherein said solid statedevices are triacs.

9. The protective circuit device of claim 8 wherein said meansgenerating a pulse trigger signal comprises a unijunction transistoroscillator.

10. The protective circuit device of claim 9 wherein said meansgenerating a pulse trigger signal includes a bridge rectifier forrectifying the associated alternating current line power supply.

11. The protective circuit device of claim 1 further including:

g. a differential transformer having a core of magnetically susceptiblemetal, a pair of like primary windings disposed about said core, and asecondary winding disposed about said core, each of said primarywindings being connected in series between one of said first pair ofterminals and one of said second pair of terminals, said pair of primarywindings under balanced current conditions therein producing a balancedtotal magnetomotive force so that the net magnetic flux in said core iszero, said pair of primary windings under leakage current conditionsproducing an unbalanced magnetomotive force to create a voltage in saidsecondary winding of said differential transformer; and

h. means responsive to said voltage in said secondary winding of saiddifierential transformer for terminating said pulse trigger signal,whereby the state of said solid state device may be changed to open thecircuit controlled thereby.

12. The protective circuit device of claim 11 wherein said meansresponsive to said voltage in said secondary winding for terminatingsaid pulse trigger signal includes a rectifier.

13. The protective circuit device of claim 12 wherein said meansresponsive to said voltage in said secondary winding for terminatingsaid pulse trigger signal also includes a stage of solid stateamplification for said voltage.

14, The protective circuit device of claim 11 wherein said meansresponsive to said voltage in said secondary winding of saiddifferential transformer includes said silicon controlled rectifier towhich current is supplied to render it conductive and latch it in saidconductive state.

k il

1. A protective circuit device for detection and control of leakage froman associated electrical load and excessive current flow from a linepower supply to an associated electrical load, comprising: a. a firstpair of terminals for connection to an external associated alternatingcurrent line power supply; b. a second pair of terminals for connectionto an external associated load; c. at least one solid state deviceconnecting one of said first pair of terminals and one of said secondpair of terminals, said solid state device having conductive andnonconductive states; d. means for changing the prevailing state of saidsolid state device including means generating a pulse trigger signal,said pulse trigger signal being transmittable to said solid state deviceto maintain one of such states, the state of said solid state devicechanging upon termination of pulse trigger signal; e. a currenttransformer having a core of magnetically susceptible metal, and only asingle primary winding connected in a series between one of said firstpair of terminals and one of said second pair of terminals, and asecondary winding, said secondary winding providing an output duringoperation which is a function of current flow in the primary winding andbelow a value established by the nominal rating of the protectivecircuit device; f. means connected to said secondary winding andresponsive to an output in excess of said value resulting from currentdrawn by the load in excess of the nominal rating of the protectivecircuit device and operative to terminate said pulse trigger signalwhereby excess current will produce an output in excess of said valueand said responsive means will terminate the pulse trigger signal tochange the state of said solid state device to open the circuitcontrolled thereby, and said responsive means including a selectivelyconductive silicon controlled rectifier to prevent generation of saidpulse trigger signal which is normally nonconductive and becomesconductive upon a predetermined output in excess of said value toterminate said pulse trigger signal and which latches in said conductivestate.
 2. The protective circuit device of claim 1 wherein said meansconnected to said secondary winding operate substantiallyinstantaneously upon occurrence of an output substantially in excess ofsaid nominal rating and, upon occurrence of an output of an excess oflesser magnitude, only after a relatively extended period of time. 3.The protective circuit device of claim 2 wherein said means connected tosaid secondary winding includes a resistance and a capacitence connectedin parallel relationship.
 4. The protective circuit device of claim 3wherein said means connected to said secondary winding includes a secondadjustable resistance to peRmit adjustment of the value of the outputwhich will operate said means to terminate said pulse trigger signal. 5.The protective device of claim 1 wherein said means connected to saidsecondary winding includes a rectifier.
 6. The protective circuit deviceof claim 5 wherein said means connected to said secondary windingincludes one stage of solid state amplification.
 7. The protectivecircuit device of claim 1 wherein a second solid state device isconnected between the other of said first pair of terminals and other ofsaid second pair of terminals, said second solid state device havingconductive and nonconductive states dependent upon said means forchanging the prevailing state of said first-mentioned solid statedevice.
 8. The protective circuit device of claim 7 wherein said solidstate devices are triacs.
 9. The protective circuit device of claim 8wherein said means generating a pulse trigger signal comprises aunijunction transistor oscillator.
 10. The protective circuit device ofclaim 9 wherein said means generating a pulse trigger signal includes abridge rectifier for rectifying the associated alternating current linepower supply.
 11. The protective circuit device of claim 1 furtherincluding: g. a differential transformer having a core of magneticallysusceptible metal, a pair of like primary windings disposed about saidcore, and a secondary winding disposed about said core, each of saidprimary windings being connected in series between one of said firstpair of terminals and one of said second pair of terminals, said pair ofprimary windings under balanced current conditions therein producing abalanced total magnetomotive force so that the net magnetic flux in saidcore is zero, said pair of primary windings under leakage currentconditions producing an unbalanced magnetomotive force to create avoltage in said secondary winding of said differential transformer; andh. means responsive to said voltage in said secondary winding of saiddifferential transformer for terminating said pulse trigger signal,whereby the state of said solid state device may be changed to open thecircuit controlled thereby.
 12. The protective circuit device of claim11 wherein said means responsive to said voltage in said secondarywinding for terminating said pulse trigger signal includes a rectifier.13. The protective circuit device of claim 12 wherein said meansresponsive to said voltage in said secondary winding for terminatingsaid pulse trigger signal also includes a stage of solid stateamplification for said voltage.
 14. The protective circuit device ofclaim 11 wherein said means responsive to said voltage in said secondarywinding of said differential transformer includes said siliconcontrolled rectifier to which current is supplied to render itconductive and latch it in said conductive state.